The invention relates to a process for mild hydrocracking hydrocarbon oils. More particularly, the invention relates to a catalytic process for increasing the yield of middle distillate products from vacuum gas oils and residuum hydrocarbon feedstocks.
In the refining of hydrocarbon oils, it is often desirable to subject the hydrocarbon oil to catalytic hydroprocessing. During hydroprocessing, particulate catalysts are utilized to promote reactions such as hydrocracking, hydrodesulfurization, hydrodenitrogenation, hydrodemetallization, and/or conversion of asphaltene compounds. One or more of such reactions may occur during hydroprocessing by contacting a particulate catalyst with a feedstock, such as a residual petroleum oil fraction or a vacuum gas oil, under conditions of elevated temperature and pressure and in the presence of hydrogen. For example, the sulfur components in the feedstock may be converted to hydrogen sulfide, nitrogen components to ammonia, asphaltenes to molecules having increased hydrogen to carbon (H-C) ratios, higher molecular weight hydrocarbons to lower molecular weight hydrocarbons and contaminant metal components to components capable of being deposited on the catalyst. Hydrocracking is typically employed in the conversion of higher molecular weight hydrocarbons to lower molecular weight hydrocarbons to convert portions of a feedstock to lighter middle distillate products, such as gasoline and diesel oils.
Refiners of hydrocarbon oils are continually challenged to convert a greater portion of the oil into lighter middle distillate products. A number of conventional conversion processes are commonly used to tackle the conversion problem. Such processes include fluid catalytic cracking, hydrocracking and coking, which all require high capital investments in the refineries, and their construction may take years to be accomplished. Because of such high costs, refiners are continually searching for conversion processes which may be utilized in existing refinery units. One such alternative process for increasing a refinery's capacity to convert feedstocks to middle distillate products is "mild hydrocracking" in existing refinery units. Such a process involves more severely hydroprocessing a feedstock to obtain increased percentages of middle distillate products while operating within the constraints of the existing refinery equipment. One of the challenges to the refiner is to discover a catalyst which will suitably perform within the design limits of the existing hydroprocessing unit. For example, a mild hydrocracking catalyst utilized in a unit formerly used for vacuum gas oil desulfurization or residuum hydrodesulfurization (i.e. a unit formerly used for less than about 10 volume percent hydrocarbon conversion) must exhibit acceptable activity, selectivity and stability, under the limiting operating conditions of the existing unit.
Therefore, an aim of the art is to provide a mild hydrocracking catalyst having a high activity, selectivity and stability. Activity may be determined by comparing the temperature at which various catalyst must be utilized under otherwise constant mild hydrocracking conditions with the same feedstock so as to produce a given percentage (between 10 volume percent and 50 volume percent) of products boiling at or below 700.degree. F. The lower activity temperature for a given catalyst, the more active such a catalyst is in relation to a catalyst of higher activity temperature. Alternatively, activity may be determined by comparing the percentages of products boiling at or below 700.degree. F. when various catalysts are utilized under otherwise constant mild hydrocracking conditions with the same feedstock. The higher the percentage of 700.degree. F.--minus product converted from the components in the feedstock boiling above 700.degree. F. for a given catalyst, the more active such a catalyst is in relation to a catalyst yielding a lower percentage of 700.degree. F.--minus product. Selectivity of a mild hydrocracking catalyst may be determined during the foregoing described activity test and is measured as that percentage fraction of the 700.degree. F.--product boiling in the range of middle distillate or midbarrel products, i.e., 300.degree. F.-700.degree. F. Stability is a measure of how well a catalyst maintains its activity over an extended period when treating a given hydrocarbon feedstock under the conditions of the activity test. Stability is generally measured in terms of the change in temperature required per day to maintain a 40 volume percent or other given conversion (less than 50 volume percent).
A typical mild hydrocracking catalyst contains hydrogenation metals on a porous refractory oxide support. Hydrogenation metals usually include Group VIB and/or Group VIII active metal components supported on an amorphous refractory oxide suport such as alumina. The catalyst may also contain one or more acidifying components. Despite the high mild hydrocracking activity of the catalysts of the prior art, catalysts of yet higher activity, selectivity, and stability are still being sought.
Accordingly, it is a major object of this invention to provide a mild hydrocracking process utilizing an active and stable catalyst, and more particularly to provide a mild hydrocracking process utilizing such a catalyst in existing refinery units to convert about 10 to about 50 volume percent of a hydrocarbon oil boiling above 700.degree. F. to middle distillate oils boiling at or below 700.degree. F.
Another object of the invention is to provide a process for removing organonitrogen and/or organosulfur compounds from a residuum hydrocarbon oil or vacuum gas oil by contracting the oil with a catalyst having a support containing silica-alumina dispersed in an alumina matrix and simultaneously converting about 10 to about 50 volume percent of the oil boiling above 700.degree. F. to middle distillate oils boiling at or below 700.degree. F.
These and other objects and advantages of the invention will become apparent from the following description.